1. Field of the Invention
The present invention relates to conductive nitride semiconductor substrates and methods for producing such substrates. Nitride semiconductors include gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and mixed crystals such as InGaN and AlInGaN. The invention is directed not to thin films deposited on underlying substrates, but to free-standing crystal substrates. Here, GaN will be mainly discussed. GaN, which has a wide bandgap, is used as a material for blue light-emitting devices.
In the related art, blue light-emitting devices such as light-emitting diodes and semiconductor lasers are produced by epitaxially growing a nitride semiconductor thin-film crystal such as InGaN, GaN, or AlInGaN on a single-crystal sapphire substrate (α-Al2O3). Sapphire has the same crystal system as GaN, namely, a hexagonal system. A c-plane GaN thin film is grown on a c-plane sapphire crystal.
A sapphire substrate, however, has insulation properties, and consequently a cathode cannot be formed on the bottom surface thereof. In addition, chips must be separated by a cutting machine because the cleavage plane of GaN differs from that of a sapphire substrate. This is disadvantageous because such chip separation takes much time and effort and also results in poor yield.
In addition, GaN differs greatly from sapphire in lattice constant. A GaN crystal grown on a sapphire substrate has high dislocation density and large bow. Accordingly, in order to use GaN itself as a substrate, the preparation of a substrate crystal of GaN has been attempted. GaN, which has a wide bandgap, is thought to be a material suitable for blue light-emitting devices. A highly conductive substrate is desired for light-emitting devices because such a substrate is advantageous in forming a cathode on the bottom surface and an anode on the top surface.
In the production of nitride semiconductor substrates, the growth of n-type nitride semiconductor crystals, which have high free electron density, has so far been researched. Currently, free-standing oxygen-doped n-type GaN substrates having a diameter of two inches can be produced.
2. Description of the Related Art
To produce light-emitting devices, metal organic chemical vapor deposition (MOCVD) is often used to form a nitride semiconductor thin film (such as a GaN, InGaN, or AlGaN thin film) on a sapphire substrate. Because MOCVD is a vapor synthesis process, the source materials are supplied in vapor phase. Nitrogen is supplied in the form of ammonia (NH3). In MOCVD, a group III element is supplied in the form of an organic metal. An organic metal of a group III element, such as gallium or indium (for example, trimethylgallium or triethylindium), and NH3 are supplied as source materials to a heated sapphire substrate.
Hydride vapor phase epitaxy (HVPE) is also frequently used as a vapor synthesis process for forming a GaN-based semiconductor thin film. In this process, a gallium boat filled with molten gallium is disposed above a susceptor, and HCl is blown therein to synthesize GaCl for use as a gallium source material. Thus, the source gases are GaCl and ammonia.
PCT International Publication No. WO99/23693 (PCT International Application No. PCT/W98/04908) (Patent Document 1) discloses a technique for forming a thick GaN crystal by forming a GaN buffer layer on a GaAs substrate on which a mask having a window diameter of 1 to 5 μm and a window pitch of 4 to 10 μm is formed and performing c-plane growth of GaN thereon at 820° C. or 970° C. by MOCVD or at 970° C., 1,000° C., 1,010° C., 1,020° C., or 1,030° C. by HVPE.
Patent Document 1 uses a mask having fine windows. FIG. 1 shows a plan view of an example of the mask. The mask is formed on an underlying substrate U. This mask has numerous small windows W regularly arranged in a large, continuous masking portion M. The underlying substrate U is exposed in the windows W. The masking portion M has a much larger area than exposed portions E (windows W). The masking portion M is a continuous thin film having the same dimensions as the underlying substrate U. The exposed portions E (openings: windows W) are larger in number but are smaller in total area.
The mechanism by which dislocations are reduced by the mask method will be described with reference to FIGS. 2A to 2G. FIGS. 2A to 2G are sectional views illustrating how a crystal is grown by the mask method. Referring to FIG. 2A, a mask M is formed by applying a mask material to the underlying substrate U and forming small windows W (exposed portions E) in a regular pattern. As GaN is grown in vapor phase, GaN crystals G are formed only in the windows W (exposed portions E). In the exposed portions E, numerous upward dislocations T occur at the boundaries between the crystals G and the underlying substrate U (see FIG. 2B).
As the growth proceeds, a portion of the crystals G growing on the exposed portions E (windows W) overgrow the masking portions (mask) M and grow across the mask M (masking portions) laterally (see FIG. 2C). As the crystals G grow laterally, the dislocations T also propagate laterally. The lateral surfaces are low-index facets F. Referring to FIG. 2D, the crystals G grow upward and laterally, thus having the shape of a truncated cone. The top surface of the truncated cone is the c-plane (C). Referring to FIG. 2E, the crystals G growing from the adjacent windows W contact each other. The dislocations T then propagate laterally and collide with each other. This causes some dislocations T to cancel each other out.
Referring to FIG. 2F, the grooves defined by the facets F are gradually filled and narrowed. Finally, the grooves defined by the facets F are completely filled, thus forming a flat surface C. The flat surface C is the c-plane. Thereafter, the growth continues at the flat c-plane surface C. The number of dislocations T is large above the windows W (exposed portions E) and is small above the masking portion (mask) M.
Patent Document 1 is important in the related art because it discloses specific data such as growth temperature and source material partial pressure. Patent Document 1 discloses that the growth temperature is 970° C., 1,000° C., 1,010° C., 1,020° C., or 1,030° C. for HVPE and is 820° C. or 970° C. for MOCVD.
For HVPE, the source materials are HCl, molten gallium, and NH3. The group III source material is GaCl, which is produced by reacting the molten gallium with HCl gas. The amounts of group III and V source materials supplied are expressed as the partial pressures PGaCl and PNH3 of GaCl and NH3, respectively. As used herein, the unit of partial pressure is Pa, where 0.1 MPa (100,000 Pa) is approximated to 1 atmospheric pressure (1 atm). More frequently, kPa (1,000 Pa) is used as a unit; in this case, 100 kPa is approximated to 1 atm. The ratio b of the group V source material to the group III source material can be expressed as the ratio of the NH3 partial pressure PNH3 to the GaCl partial pressure PGaCl. The ratio of the group V source material to the group III source material is referred to as the V/III ratio b, as defined as b=PNH3/PGaCl.
According to Patent Document 1, the GaN crystal formed by the mask method has a resistivity r of 0.005 to 0.08 Ωcm.
The substrate temperature and the V/III ratio b during growth are important conditions responsible for crystal growth. FIG. 22 lists the growth temperatures and the V/III ratios b of all of Patent Document 1 to 11 described below, where the horizontal axis indicates the substrate temperature (° C.) and the vertical axis indicates the V/III ratio b. The coordinates of the substrate temperatures and V/III ratios b of the GaN crystals shown in the patent documents are marked with dots. The black dots indicate GaN crystals formed by HVPE, and the circled black dots indicate GaN crystals formed by MOCVD. The numbers beside the black dots and the circled black dots are the number of the patent documents. The growth conditions for MOCVD disclosed in the examples in Patent Document 1 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the trimethylgallium (TMG) partial pressure PTMG, and the V/III ratio b are shown below in the above order:
  970° C.20 kPa0.2 kPa100 times  970° C.25 kPa0.2 kPa100 times  820° C.20 kPa0.3 kPa 67 times  970° C.20 kPa0.2 kPa100 times1,000° C.20 kPa0.4 kPa 50 times  970° C.25 kPa0.5 kPa 50 times
The substrate temperature for MOCVD in Patent Document 1 is 820° C. to 1,000° C., and the V/III ratio b is 50 to 100 times. These examples are indicated by the five circled black dots in the middle left of FIG. 22. The third example, namely, 820° C. and 67 times, is omitted from FIG. 22 because the temperature is excessively low and deviates from FIG. 22.
The growth conditions for HVPE disclosed in the examples in Patent Document 1 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the HCl partial pressure PHCl, and the V/III ratio b are shown below in the above order:
  970° C.25 kPa  2 kPa12.5 times  970° C.25 kPa2.5 kPa  10 times  970° C.25 kPa0.5 kPa  50 times1,000° C.20 kPa  2 kPa  10 times  950° C.25 kPa  2 kPa 12.5 times1,020° C.25 kPa  2 kPa 12.5 times1,000° C.25 kPa  2 kPa 12.5 times1,010° C.25 kPa  2 kPa 12.5 times1,030° C.25 kPa  2 kPa12.5 timesThe substrate temperature for HVPE in Patent Document 1 is 950° C. to 1,030° C., and the V/III ratio b is 10 to 50 times.
Japanese Patent No. 3788037 (Japanese Patent Application No. 10-171276, Japanese Unexamined Patent Application Publication No. 2000-012900) (Patent Document 2) provides a free-standing GaN substrate having a diameter of 20 mm or more, a thickness of 70 μm or more, and a deflection (bow) of 0.55 mm or less on a 50 mm diameter wafer basis by forming a mask having a staggered pattern of fine windows on a GaAs substrate, growing a thick GaN film thereon by HVPE while maintaining the c-plane, and removing the GaAs substrate. A central deflection (bow) of 0.55 mm on a 50 mm diameter wafer basis is equivalent to a radius of curvature R of about 600 mm=0.6 m, meaning that the curvature is large.
In Patent Document 2, the growth temperature for HVPE is 970° C., 1,020° C., or 1,030° C., the GaCl partial pressure PGaCl is 1 or 2 kPa (0.01 to 0.02 atm), and the NH3 partial pressure PNH3 is 4 or 6 kPa. Patent Document 2 discloses that a crystal formed at a GaCl partial pressure PGaCl of 1 kPa is impractical because it has a flat surface but has large bow and large internal stress and cracks easily, and it cannot be formed to a thickness of 70 μm or more.
Conversely, according to Patent Document 2, a crystal formed at a GaCl partial pressure PGaCl of 2 kPa has a rough surface but has small bow and small internal stress. The NH3 partial pressure is 6, 12, or 24 kPa. The WM ratio b is 3, 6, or 12. The radius of curvature is about 1 m. The resistivity is 0.0035 to 0.0083 Ωcm. This crystal is n-type.
The growth conditions disclosed in the examples in Patent Document 2 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the GaCl partial pressure PGaCl, and the V/III ratio b are shown below in the above order:
1,030° C. 4 kPa1 kPa 4 times1,030° C. 6 kPa1 kPa 6 times  970° C. 6 kPa2 kPa 3 times  970° C. 6 kPa1 kPa  6 times  970° C. 6 kPa1 kPa  6 times1,020° C. 6 kPa2 kPa 3 times1,020° C. 6 kPa2 kPa  3 times1,030° C. 6 kPa1 kPa  6 times  970° C. 6 kPa2 kPa 3 times  970° C.12 kPa2 kPa 6 times  970° C. 24 kPa2 kPa12 timesAll examples are formed by HVPE. The substrate temperature is 970° to 1,030° C., and the V/III ratio b is 3 to 12 times. The growth conditions (substrate temperature and V/III ratio b) of Patent Document 2 are indicated by the eleven black dots in the lower left of FIG. 22.
Japanese Patent No. 3788041 (Japanese Patent Application No. 10-183446, Japanese Unexamined Patent Application Publication No. 2000-022212) (Patent Document 3) proposes a method for producing a free-standing single-crystal GaN substrate by forming, on a GaAs substrate, a mask having a dot pattern of windows arranged at predetermined intervals in the [11-2] direction and shifted by half the pitch in the [−110] direction, a mask having a stripe pattern of windows extending in the [11-2] direction, or a mask having a stripe pattern of windows extending in the [−110] direction; forming a buffer layer; epitaxially growing GaN by HVPE while maintaining the c-plane; and removing the substrate and the mask.
The technique proposed in Patent Document 3 reduces dislocations T by forming a mask having numerous small windows arranged at a narrow pitch in two orthogonal directions on the underlying substrate U, as shown in FIG. 1, and growing GaN in vapor phase. The GaCl partial pressure PGaCl is 1 kPa (0.01 atm) or 2 kPa (0.02 atm). Patent Document 3 discloses that a GaN crystal formed at a GaCl partial pressure PGaCl of 1 kPa has a flat surface but has large internal stress and large bow and cracks easily and that a GaN crystal formed at a GaCl partial pressure PGaCl of 2 kPa has a rough surface but has small internal stress and small bow and does not crack easily.
Patent Document 3 also discloses that a crystal formed at a growth temperature of 1,020° C. or 1,030° C. has a flat surface but has large internal stress and cracks easily.
Patent Document 3 also discloses that a thick GaN crystal formed at a growth temperature of 970° C. and a GaCl partial pressure PGaCl of 2 kPa has a rough surface but has small internal stress and small bow. The NH3 partial pressure PNH3 is 6 to 12 kPa.
In summary, according to Patent Document 3, the conditions for producing a GaN crystal that has a rough surface but has small bow and small internal stress and does not crack easily are a growth temperature of 970° C., a GaCl partial pressure of 2 kPa, a NH3 partial pressure of 6 to 12 kPa, and a V/III ratio b of about 3 to 6. The crystal has a resistivity of 0.01 to 0.017 Ωcm and is n-type.
The HVPE growth conditions disclosed in the examples in Patent Document 3 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the GaCl partial pressure PGaCl, and the V/III ratio b are shown below in the above order:
1,030° C. 4 kPa1 kPa 4 times1,030° C. 6 kPa1 kPa 6 times  970° C. 6 kPa2 kPa 3 times  970° C. 6 kPa1 kPa 6 times  970° C. 6 kPa1 kPa 6 times1,020° C. 6 kPa2 kPa 3 times1,020° C. 6 kPa2 kPa 3 times1,030° C. 6 kPa1 kPa 6 times  970° C. 6 kPa2 kPa 3 times  970° C. 12 kPa2 kPa 6 times  970° C.24 kPa2 kPa12 timesAll examples are formed by HVPE. The substrate temperature is 970° to 1,030° C., and the V/III ratio b is 3 to 12 times. The growth conditions (substrate temperature and V/III ratio b) of Patent Document 3 are indicated by the eleven black dots in the lower left of FIG. 22.
In PCT International Publication No. WO98/47170 (PCT International Application No. PCT/JP 98/01640) (Patent Document 4), two or three epitaxial lateral overgrowth (ELO) masks are formed so as to be alternately superimposed to reduce dislocations, and a silicon-doped n-type GaN crystal is grown by MOCVD or HVPE while maintaining the c-plane. According to Patent Document 4, dislocations can be reduced by forming two or three ELO masks such that windows are shifted from each other because the dislocation density is higher above the windows and is lower above the masks. Patent Document 4 discloses that a V/III ratio b of 30 to 2,000 times is appropriate for MOCVD.
In the examples in Patent Document 4, MOCVD is used, and source gases are used in a V/III ratio b of 12,000 times, 2,222 times, 1,800 times, 1,500 times, 800 times, or 30 times. Patent Document 4 discloses that the preferred growth temperature is 950° C. to 1,050° C. The conditions for HVPE are not disclosed. In FIG. 22, lines are drawn along the temperature range of 950° C. to 1,050° C. at b=2,222 times, 1,800 times, 1,500 times, 800 times, and 30 times and are marked with circled black dots. The n-type dopant is silicon. Silicon serves as an n-type dopant because a silicon atom replaces a gallium atom to release one free electron. Silane gas (SiH4) is used for doping. First, truncated crystals are formed above the windows of the ELO masks by MOCVD, and the process is switched to HVPE immediately before the truncated crystals combine above the ELO masks.
EPC Publication No. EP0942459 A1 (EPC Application No. 9891274.8) (Patent Document 5) is substantially the same as Patent Document 4 and proposes a technique for reducing dislocations using two or three ELO masks. The growth conditions, including the substrate temperature, the group III source material partial pressure, and the group V source material partial pressure, are the same as those of Patent Document 4 and therefore will not be described. Although not shown in FIG. 22, the growth conditions of Patent Document 5 are equivalent to those of Patent Document 4.
Japanese Patent No. 3788104 (Japanese Unexamined Patent Application Publication No. 2000-044400, Japanese Patent Application No. 11-144151, priority claim based on Japanese Patent Application No. 10-147716) (Patent Document 6) first proposed a method for producing an n-type GaN substrate by doping GaN with oxygen (O) as an n-type dopant. Oxygen may serve as an n-type dopant because it can release one free electron by replacing nitrogen. However, it is unknown what level oxygen forms in GaN, and it is therefore uncertain whether oxygen serves as an n-type dopant unless a doping test is carried out in practice. Patent Document 6 does not specifically disclose substrate temperature or source material partial pressure, and they are not shown in FIG. 22.
In Patent Documents 4 and 5, a crystal is doped with silicon, which serves as an n-type dopant, using silane gas (SiH4). The use of a large amount of silane gas for growth of an n-type substrate is hazardous because it has the risk of explosion. Patent Document 6 has found that oxygen forms a shallow donor level in GaN. If a GaN crystal is grown by HVPE on a GaAs substrate on which an ELO mask is formed using a source gas, such as NH3 or HCl, to which water is added, the crystal is doped with oxygen from the source gas during c-plane growth to form a donor level and produce n-type carriers, thus making the crystal n-type. In addition, according to Patent Document 6, the activation rate is 100% over a wide range of concentration. Thus, Patent Document 6 first revealed that oxygen can serve as an n-type dopant advantageous for a thick crystal such as a substrate. Patent Document 6, however, only first pointed out that oxygen can serve as an n-type dopant and does not recognize anisotropy in doping.
Japanese Patent No. 3826825 (Japanese Unexamined Patent Application Publication No. 2002-373864, Japanese Patent Application No. 2002-103723, priority claim based on Japanese Patent Application No. 2001-113872) (Patent Document 7) reveals that oxygen doping of GaN has significant anisotropy, that is, the selectivity that oxygen is not easily absorbed through the c-plane ((0001) plane) and is easily absorbed through planes other than the c-plane. As shown in FIG. 17, Patent Document 7 proposes a technique for growing a crystal in the c-axis direction as a whole while forming numerous non-c-plane facets F in the surface thereof so that oxygen is absorbed into the crystal through the non-c-plane facets F, as shown in FIG. 17, or growing a crystal on a GaN underlying substrate having a non-c-plane surface (hkmn) (≠(0001) plane) while doping it with oxygen through the non-c-plane surface, as shown in FIG. 18. Thus, Patent Document 7 first revealed significant anisotropy in oxygen doping.
The HVPE growth conditions disclosed in an example in Patent Document 7 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the HCl partial pressure PHCl, and the V/III ratio b are shown below in the above order:
1,020° C.20 kPa1 kPa20 timesThe coordinates of 1,020° C. and 20 times in FIG. 22 are indicated by the black dot marked with “7”.
Japanese Unexamined Patent Application Publication No. 2001-102307 (Japanese Patent Application No. 11-273882) (Patent Document 8) proposes a novel method for reducing dislocation density that is quite different from ELO (see FIG. 1 and FIGS. 2A to 2G; that is, as shown in FIG. 2G, a crystal is grown while maintaining a flat c-plane surface). In Patent Document 8, as shown in FIG. 3, numerous pits P of varying sizes defined by facets F are intentionally formed under appropriately controlled growth conditions, and the facets F are maintained to the end of the growth without filling the pits P. This method is referred to as facet growth since the facets F are maintained to the end without filling the pits P. Although the pits P are six- or twelve-sided pyramids, six-sided pyramid pits are shown here for brevity.
As shown in the perspective view of the pits P in FIG. 4 and the plan view of the pits P in FIG. 5, as a crystal is grown while maintaining the recesses (pits P) defined by the facets F, it grows in a direction V normal to the facets F inside the pits P. Because dislocations T propagate in the growth direction, they propagate in the direction V normal to the facets F. During the facet growth, the dislocations T cluster at boundaries B. Thus, clusters of dislocations T are formed under the boundaries B (planar defects PD).
As the facet growth proceeds, the dislocations T further cluster at the bottoms of the pits P. Clusters of numerous dislocations T (linear clusters of defects H) are formed at the bottoms of the pits P. Even through the total number of dislocations T remains nearly the same, the dislocation density is lower at other portions because the dislocations T cluster into the planar defects PD and the linear defects H. Unlike ELO (see FIG. 1 and FIGS. 2A to 2G), this method provides the effect of reducing dislocations from the middle stage to the final stage of the growth. Thus, this is a completely novel method for reducing dislocation density. This method is referred to as facet growth since it maintains facets to reduce dislocations by the effect of the facets.
Because it is uncertain where the pits P (recesses) are formed in the technique in Patent Document 8, this is referred to as random facet growth to distinguish it from the later improved versions. The resultant crystal has noticeable surface irregularities.
The growth conditions disclosed in the examples in Patent Document 8 (random facet growth) are as follows.
The growth temperature T, the NH3 partial pressure PNH3, the HCl partial pressure PHCl, and the V/III ratio b are shown below in the above order:
1,050° C.20 kPa0.5 kPa40 times1,000° C.30 kPa  2 kPa15 times1,050° C.20 kPa0.5 kPa40 times1,020° C.20 kPa  1 kPa20 times1,000° C.30 kPa  2 kPa15 times1,000° C.40 kPa  3 kPa13 times  980° C.40 kPa  4 kPa10 timesIn the growth conditions of Patent Document 8, the substrate temperature is 980° C. to 1,050° C., and the V/III ratio b is 10 to 40 times. The V/III ratio b is high. Seven black dots having the substrate temperatures and the V/III ratios b shown above as the coordinates thereof are distributed in the center of the lower half of FIG. 22.
In Patent Document 8, the positions where facet pits are formed are randomly determined because the substrate has no anisotropy or specificity; thus, this method is referred to as random facet growth. Because the pit positions are randomly determined, it is difficult to fabricate devices on the substrate. In addition, clustered dislocations can be redispersed as the crystal grows because of the absence of local specificity. Because devices are fabricated on the substrate, it will be more advantageous if the positions where facet pits are formed can be designated in advance. In addition, the dislocation density will be further reduced if the dislocations can be confined and prevented from being redispersed.
As shown in FIG. 6, Japanese Patent No. 3864870 (Japanese Unexamined Patent Application Publication No. 2003-165799, Japanese Patent Application No. 2002-230925, priority claim based on Japanese Patent Application No. 2001-284323) (Patent Document 9) uses a mask having an isolated dot pattern of masking portions M regularly arranged on the underlying substrate U. Unlike the ELO mask shown in FIG. 1 and FIGS. 2A to 2G, the region where the underlying substrate U is exposed (exposed portion E) is much larger than the masking portion (mask portions) M. The sum of the spacing w and the diameter s of the masking portions M is the pitch p. The spacing w is much larger than the diameter s, and the pitch p of the masking portions M is much larger than that of an ELO mask. GaN is grown in vapor phase on the masked underlying substrate U. The growth starts from the exposed portion E, and a thin film is formed on the exposed portion E. Because the growth is delayed on the masking portions M, recesses (facet pits P) whose bottoms are defined by the masking portions M are formed.
Facet growth using a dot mask will now be described with reference to FIGS. 7, 8, and 9A to 9F. Referring to FIG. 9A, an isolated dot pattern of masking portions M is formed on the underlying substrate U. Referring to FIG. 9B, as GaN is grown in vapor phase, a crystal G grows only on the exposed portion E of the underlying substrate U; it does not grow on the masking portions M. Referring to FIG. 9C, the crystal G grows upward above the exposed portion E. The inclined surfaces are low-index facets F. Referring to FIG. 9D, six- or twelve-sided pyramid facet pits P are formed whose bottoms are defined by the masking portions M and whose inclined surfaces are defined by the facets F. Referring to FIG. 9E, the crystal G overgrows the masking portions M. These portions are closed defect cluster regions H where dislocations cluster at high density. The portions below the facets F are single-crystal low-dislocation-density concomitant regions Z. The flat surface is the c-plane (C). The portion below the c-plane is a single-crystal low-dislocation-density remaining region Y.
As shown in the perspective view of the crystal G in FIG. 7 and the plan view of FIG. 8, facet pits P defined by facets F having the shape of an inverted cone like a flower petal are arranged in the surface of the crystal G in two orthogonal directions. The portions corresponding to the stalk are the closed defect cluster regions H where dislocations cluster. The portions corresponding to the root are the masking portions M. The flat surface is the c-plane. The portion under the c-plane is a low-dislocation-density region (Y). The portions below the facets F are also low-dislocation-density regions (Z). This mask is referred to as a dot mask to distinguish it from others. This method is referred to as dot facet growth.
The facet pits P, as described above, have the effect of causing dislocations in the facets F to cluster at the boundaries B and further cluster at the bottoms of the pits P. The bottoms of the pits P (above the masking portions M) are the closed defect cluster regions H where dislocations cluster. The closed defect cluster regions H are “closed” because the clustered dislocations are no longer redispersed. The other portions are the single-crystal low-dislocation-density concomitant regions Z (formed below the facets F) having a low-dislocation-density and the single-crystal low-dislocation-density remaining region Y (formed below the c-plane). The regions Z and Y have low dislocation density.
Patent Document 9 first introduced the concepts of the closed defect cluster regions H, the single-crystal low-dislocation-density concomitant regions Z, and the single-crystal low-dislocation-density remaining region Y. The closed defect cluster regions H are formed above the masking portions M. The single-crystal low-dislocation-density concomitant regions Z are formed above the exposed portion E, where the masking portions M are not formed, beside (so as to be attached to) the masking portions M. The single-crystal low-dislocation-density remaining region Y is formed above the exposed portion E in a region remote from the masking portions M. The single-crystal low-dislocation-density concomitant regions Z are formed beside (so as to be attached to) the masking portions M because they are formed below the facets F, which are formed diagonally with respect to the masking portions M so as to cover the exposed portion E. The single-crystal low-dislocation-density remaining region Y is formed in a region remote from the masking portions M because they are surrounded by the single-crystal low-dislocation-density concomitant regions Z. Unlike an ELO mask, the mask used for facet growth does not have fine windows arranged at a small pitch, but has a dot pattern (such as circles or squares) of masking portions M (see FIG. 6) of considerable size in a large exposed portion.
For an ELO mask (see FIGS. 1 and 2A to 2G), the masking portion M is a single continuous region larger than the exposed portions E (windows W), and the numerous exposed portions E (windows W) are small (1 to 2 μm in diameter), are arranged at a small pitch (2 to 6 μm), and have a total area smaller than that of the masking portion M.
Conversely, for the mask serving as the basis of the facet pits P in Patent Document 9, as shown in FIG. 6, the exposed portion E is larger than the masking portions M. The exposed portion E is a single continuous region. The masking portions M are large in number but have a total area smaller than that of the exposed portion E. As shown in FIGS. 7, 8, and 9A to 9F, because the facets F are formed above the exposed portion E and low-dislocation-density high-quality regions, namely, the single-crystal low-dislocation-density concomitant regions Z, are formed directly below the facets F, it is essential to form a large exposed portion E. The diameter of the masking portions M is considerably large (20 to 100 μm in diameter). The bottoms of the facet pits P are located above the masking portions M. The facet pits P collect and trap dislocations at the bottoms thereof and do not release the dislocations. This mask is characterized in that the closed defect cluster regions H are formed above the masking portions M and the low-dislocation-density regions Z and Y are formed therearound (see FIGS. 8 and 9A to 9F). The low-dislocation-density regions Z and Y are formed above the exposed portion E, where the masking portions M are not formed. The regions Z are formed directly below the facets F, and the region Y is formed directly below the c-plane growth portion. The regions Z and Y are formed of a single crystal and have low dislocation density. Thus, concentric HZY structures are formed around the dot masking portions M. This relationship is opposite to that of an ELO mask, in which high-dislocation-density regions (H) are formed above the exposed portions E (windows W) and low-dislocation-density regions (Z, Y) are formed above the masking portion M.
The growth conditions of the examples in Patent Document 9 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the HCl partial pressure PHCl, and the V/III ratio b are shown below in the above order:
1,050° C.30 kPa  2 kPa  15 times1,030° C.30 kPa2.5 kPa  12 times1,010° C.20 kPa2.5 kPa  8 times1,030° C.25 kPa2.5 kPa  10 times1,050° C.30 kPa2.5 kPa  12 times1,030° C.25 kPa  2 kPa12.5 times1,030° C.25 kPa  2 kPa12.5 timesThe growth temperature is 1,010° C. to 1,050° C., and the VIII ratio b is 8 to 15 times.
These examples are indicated by the seven black dots marked with the number “9” in the center of the lower half of FIG. 22.
In Patent Document 9, because the mask has a regularly distributed isolated dot pattern, the closed defect cluster regions H are formed above the dots (masking portions), and the single-crystal low-dislocation-density concomitant regions Z and the single-crystal low-dislocation-density remaining region Y are formed therearound above the exposed portion E. The dispersed closed defect cluster regions H are disadvantageous in fabricating devices such as semiconductor lasers or light-emitting diodes on the substrate.
Accordingly, in Japanese Patent No. 3801125 (Japanese Unexamined Patent Application Publication No. 2003-183100, Japanese Patent Application No. 2002-269387, priority claim based on Japanese Patent Application No. 2001-311018) (Patent Document 10), as shown in FIG. 10, a mask having a regularly spaced parallel stripe pattern of masking portions M is formed on the underlying substrate U, and GaN is grown thereon by facet growth. The sum of the width s of the masking portions M and the width w of the exposed portions E is the pitch p (p=s+w). The widths s and w in this method are larger than the pitch or spacing in ELO. The width w is much larger than the width s. GaN is grown in vapor phase on the underlying substrate U.
As shown in the plan view of FIG. 11 and the perspective view of FIG. 12, numerous parallel ridge-and-valley crystals having flat top faces are formed. Parallel crystal defect cluster regions H are formed above the masking portions M, and parallel low-dislocation-density single-crystal regions Z and c-plane growth regions Y are formed above the exposed portions E. The regions where dislocations cluster above the parallel masking portions M are referred to as the crystal defect cluster regions H. The regions continuously grown below the facets F adjacent to the crystal defect cluster regions H are referred to as the low-dislocation-density single-crystal regions Z. The c-plane growth regions Y may or may not be formed between the adjacent low-dislocation-density single-crystal regions Z.
Whereas Patent Document 9 uses the term “closed” to emphasize that the defect regions H are closed because they are formed above the isolated dot pattern of masking portions M, the term “closed” is inappropriate for the regions H formed above the stripe pattern of masking portions M because they are not closed at the ends thereof; thus, the regions formed above the masking portions M are referred to as the crystal defect cluster regions H. These regions are the same as the closed defect cluster regions H in Patent Document 9 and are therefore denoted by the same symbol H. In addition, the term “concomitant” is inappropriate for the regions Z because the exposed portions E spread continuously and the regions Z are not necessarily concomitant with the regions H; thus, the regions Z are referred to as the low-dislocation-density single-crystal regions Z. These regions are the same as the single-crystal low-dislocation-density concomitant regions Z in Patent Document 9 and are therefore denoted by the same symbol Z. For the dot mask in Patent Document 9, a remaining portion occurs necessarily if osculating circles of equal radius are drawn about the masking portions M. Accordingly, the single-crystal low-dislocation-density remaining region Y occurs necessarily in Patent Document 9. In Patent Document 10, on the other hand, the regions Y may or may not be formed because the mask has a parallel stripe pattern. The regions Y are formed where the c-plane appears. Because the regions Y occur in the c-plane, they are referred to as the c-plane growth regions Y in Patent Document 10. These regions are the same as the single-crystal low-dislocation-density remaining region Y and are therefore denoted by the same symbol Y.
The c-plane growth regions Y may disappear depending on the manner of growth. As shown in the plan view of FIG. 13 and the perspective view of FIG. 14, ridge-and-valley crystals having sharp ridges may be formed. The parallel crystal defect cluster regions H are formed above the masking portions M, thus forming the valleys. The parallel low-dislocation-density single-crystal regions Z are formed above the exposed portions E adjacent thereto. The ridges formed by the facets F are sharp and have no c-plane portion. The c-plane growth regions Y are lost; that is, the . . . ZHZH . . . structure is formed.
Stripe facet growth will now be described with reference to FIGS. 15A to 15F.
Referring to FIG. 15A, a parallel stripe pattern of masking portions M is formed on the underlying substrate U. The pitch p of the masking portions M (20 to 2,000 μm) is much larger than the pitch of the mask windows w in FIGS. 1 and 2A to 2G (about 2 to 6 μm). The exposed portions E are larger than the masking portions M. Referring to FIG. 15B, as GaN is grown in vapor phase, crystals G grow only on the exposed portions E of the underlying substrate U; they do not grow on the masking portions M. Referring to FIG. 15C, the crystals G grow upward above the exposed portions E. The inclined faces are low-index facets F. The crystals G are formed as parallel stripes separated by the masking portions M. Referring to FIG. 15D, parallel V-grooves whose bottoms are defined by the masking portions M are formed by the parallel inclined faces inclined in opposite directions. The opposite inclined faces are the facets F inclined at the same angle in opposite directions. The flat faces between the adjacent masking portions M are the c-plane (C).
Referring to FIG. 15E, the crystals G overgrow the masking portions M. These portions are the crystal defect cluster regions H where dislocations cluster at high density. Referring to FIG. 15F, the crystals G grow further. The crystal defect cluster regions H above the masking portions M grow upward while substantially maintaining their areas. The parallel facets F become larger. The portions directly below the facets F are the low-dislocation-density single-crystal regions Z. The boundaries between the crystal defect cluster regions H and the low-dislocation-density single-crystal regions Z are crystal boundaries K. The crystal boundaries K confine dislocations within the crystal defect cluster regions H.
The flat faces formed midway between the masking portions M are the c-plane (C). The portions below the c-plane are the c-plane growth regions Y. The c-plane becomes gradually narrower. The pitch of the stripe pattern of crystals G is equal to the mask pitch p, which is the sum the width s of the masking portions M and the width w of the exposed portions E (p=s+w). As the crystal growth proceeds, as shown in FIG. 16A, parallel crystals grow like a mountain range, with the crystal defect cluster regions H forming the bases of the mountains and the c-plane forming the ridges of the mountains. The c-plane portions (C), corresponding to the mountaintops, become narrower. The portions directly below the facets F are the low-dislocation-density single-crystal regions Z, and the portions directly below the c-plane are the c-plane growth regions Y.
The crystals G may grow upward while maintaining their shapes, or may grow further into the shape of parallel mountains having sharp peaks, as shown in FIG. 16B. In this case, the c-plane disappears, and the c-plane growth regions Y also disappear.
In Patent Document 10, the . . . ZHZYZHZYZH . . . structure or the . . . ZHZHZH . . . structure is formed. The crystal defect cluster regions H have dislocations concentrated therein, and the low-dislocation-density single-crystal regions Z and the c-plane growth regions Y are formed of a single crystal and have low dislocation density.
The method of Patent Document 10 is referred to as stripe facet growth because the parallel crystal defect cluster regions H are formed by forming a mask having a parallel stripe pattern. In this case, devices such as semiconductor lasers or light-emitting diodes can be easily fabricated because the low-dislocation-density single-crystal regions Z extend in a straight line.
Facet growth and ELO are totally different methods, and the shapes, dimensions, and effects of the masks are also different. An ELO mask, which has windows distributed in a staggered pattern, can be clearly distinguished from a stripe mask because they have different shapes and dimensions. An ELO mask has numerous small windows having a diameter of 1 to 2 μm and distributed at a pitch of about 2 to 6 μm. A stripe mask has a width s of about 10 to 300 μm and a pitch p of about 20 to 2,000 μm. A stripe mask is a coarse mask, for example, having a width s of 50 μm and a pitch p of 500 μm.
In dot facet growth and stripe facet growth, dislocations are concentrated in the crystal defect cluster regions H above the masking portions M and are no longer redispersed because they are confined by the crystal boundaries K. The regions Z and Y adjacent to the regions H have low dislocation density and are formed of a single crystal. These portions may be used as portions where a device current flows.
A GaN crystal can be used to form a cavity mirror of a laser by natural cleavage because it cleaves in a {1-100} direction. A cathode can be formed on the bottom surface of the crystal because it is made n-type by oxygen doping so that a current flows therethrough. In this respect, a GaN crystal is superior to a sapphire substrate.
In ELO, high-dislocation-density regions are formed above small exposed portions, and a low-dislocation-density region is formed above a large masking portion. In stripe facet growth, on the other hand, low-dislocation-density regions are formed above wide exposed portions, and high-dislocation-density regions are formed above narrow masking portions.
The growth conditions of the examples (all by HVPE) in Patent Document 10 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the HCl partial pressure PHCl and the V/III ratio b are shown below in the above order:
1,050° C.30 kPa  2 kPa  15 times1,030° C.30 kPa2.5 kPa  12 times1,050° C.30 kPa  2 kPa  15 times1,010° C.20 kPa2.5 kPa  8 times1,030° C.25 kPa  2 kPa12.5 times1,030° C.25 kPa2.5 kPa  10 times
In Patent Document 10, the substrate temperature is 1,010° C. to 1,050° C., and the V/III ratio b is 8 to 15 times. The growth conditions of Patent Document 10 are indicated by the six dots marked with the number “10” in the center of the lower half of FIG. 22.
Japanese Unexamined Patent Application Publication No. 2005-306723 (Japanese Patent Application No. 2005-075734, priority claim based on Japanese Patent Application No. 2004-085372) (Patent Document 11) proposes a method for producing an iron-doped GaN substrate by growing an iron-doped GaN crystal on a sapphire (0001) substrate by MOCVD using H2, TMG, and ammonia as source gases and (C5H5)2Fe as a dopant, or by HVPE using H2, HCl, molten gallium, and ammonia as source materials and (C5H5)2Fe as a dopant. Patent Document 11 is intended to produce a semi-insulating substrate by iron doping.
The growth conditions of an example (MOCVD) in Patent Document 11 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the TMG partial pressure PTMG, and the V/III ratio b are shown below in the above order:
1,000° C.15 kPa0.3 kPa50 timesThis example is indicated by the circled black dot at 1,000° C. and 50 times in the center of FIG. 22.
The growth conditions of an example (HVPE) in Patent Document 11 are as follows.
The growth temperature Tq, the NH3 partial pressure PNH3, the HCl partial pressure and the V/III ratio b are shown below in the above order:
1,000° C.15 kPa0.3 kPa50 timesThis example is indicated by the black dot at 1,000° C. and 50 times in the center of FIG. 22.
The substrates that have so far been discussed are n-type GaN substrates used as substrates for blue light-emitting diodes and semiconductor lasers and semi-insulating (SI) GaN substrates used as substrates for field-effect transistors (FET). AlInGaN substrates containing small amounts of aluminum and indium have also been produced for use as substrates for light-emitting devices. These substrates allow a high-density current to flow therethrough because they are n-type and have high conductivity. The dopant is silicon (Si) or oxygen (O).
For light-emitting devices, dislocations may lead to deterioration because a high-density current flows through the substrate. Accordingly, a low dislocation density is desired in view of inhibiting deterioration. However, there are other problems.
A substrate having high dislocation density is undesirable because it causes current leakage. Fewer dislocations are desirable because GaN, InGaN, or AlGaN thin films having a regular lattice structure are formed in layers on the substrate. In addition, a substrate having little bow and low cracking ratio is desirable because semiconductor devices are fabricated thereon. Accordingly, as a conductive substrate, a substrate having high conductivity, little bow, low dislocation density, and low cracking ratio is strongly desired.
The GaN substrates mentioned as the related art (Patent Documents 1 to 10) have low resistivity and are n-type.
Patent Document 1 discloses that the resistivity is 0.005 to 0.08 Ωcm. Patent Document 2 discloses that the resistivity is 0.0035 to 0.0083 Ωcm. Patent Document 3 discloses that the resistivity of the GaN substrate is 0.01 to 0.017 Ωcm.
In Patent Documents 1 to 3, it is assumed that vacancies of the group V element form a donor level or an n-type dopant element contained in the source gases is introduced because they do not disclose that an n-type dopant is introduced.
Patent Document 4 does not specify resistivity. Because this intends to form a low-resistivity n-type GaN substrate using silicon as a dopant, it is assumed that the resistivity is lower than those of Patent Documents 1 to 3. From the descriptions thereof, it is assumed that the upper limit of the GaN crystals of the related art is about 0.08 Ωcm and the lower limit thereof is about 0.005 Ωcm.
ELO cannot sufficiently reduce dislocations. Facet growth, proposed in Patent Documents 8 and 9, have the effect of reducing dislocation density. Currently known methods for forming conductive GaN include oxygen doping and silicon doping. For oxygen doping, as in Patent Documents 6 and 7, water or oxygen may be mixed in a source gas. These are safe substances. For silicon doping, as in Patent Documents 4 and 5, silane (SiH4) gas must be supplied. Silane gas is a hazardous gas and therefore should not be used in large quantities. In addition, it is uncertain whether or not the absorption of SiH4 gas has plane dependence. There is no research as to whether or not silicon doping has anisotropy for the c-plane and the m- or a-plane, or generally a facet plane (hereinafter abbreviated to “f-plane”). If silicon has the same plane orientation dependence as oxygen, the dosage of the n-type impurity cannot be made uniform. However, if silicon and oxygen have different plane dependence, the resistivity may be complementarily made uniform.
Facet growth is effective in reducing dislocation density. By causing dislocations to cluster into closed defect cluster regions above the masking portions, the dislocation density can be reduced in other portions. In addition, the substrate has little bow and low cracking ratio because it has a rough surface.
Nevertheless, facet growth has some disadvantages. One of the disadvantages is anisotropy in oxygen absorption during oxygen doping. The use of facet growth for forming an oxygen-doped n-type crystal results in significant plane orientation dependence in doping efficiency. Patent Document 7 discloses that the efficiency of oxygen doping is lowest in the c-plane and that the amount of oxygen absorbed through the facets is 50 times or more that through the c-plane. Facet growth portions and c-plane growth portions are mixed in the crystal formed by facet growth. The oxygen concentration is lower in the c-plane because it absorbs little oxygen and is higher in the facets because the amount of oxygen absorbed through the facets is 50 times or more that through the c-plane. A crystal formed by facet growth has uneven oxygen concentration because it includes c-plane portions. If devices are fabricated using such crystals as substrates, the devices have significant variations in conductivity.
If a crystal is formed by c-plane growth, rather than by facet growth, it has high resistivity because little oxygen is absorbed. In addition, the crystal formed by c-plane growth, in which the crystal is grown while maintaining a flat surface, cracks and splits off easily, whereas a crystal formed by facet growth does not crack easily and is robust. Thus, an n-type substrate having low cracking ratio, little bow, and uniform resistivity is desired.